1. Field of the Invention
The present invention relates to a null symbol detection device used for receivers in a digital broadcasting system in which a null symbol repetition period or a null symbol width is different depending on transmission modes.
2. Discussion of the Related Art
In accordance with some standards for terrestrial digital broadcasting system, a null symbol with smaller transmission power than those of other symbols including broadcasting data is repeatedly transmitted at a fixed period depending on transmission modes.
Conventional null symbol detection devices detect the symbol width of a null symbol. Such method is disclosed in, e.g., Japanese Unexamined Patent Publication No. 11-27333 (1999) and U.S. patent application Publication No. 2002-0042661. FIG. 1 is a block diagram illustrating the structural example of a conventional null symbol detection device. This null symbol detection device has a level detector 1, an interval width counter 2, a clock generator 3, a null interval width base counter 4, an interval width evaluator 5 and a synchronous signal generator 6.
The level detector 1 compares the signal level of an inputted baseband signal with a threshold to output an H-level signal when the signal level of the inputted baseband signal is less than the threshold and to output an L-level signal when the signal level thereof is equal to or more than the threshold. The interval width counter 2 detects the time width or the clock number of the interval during which the level detector 1 outputs an H-level signal. The clock generator 3 generates a reference clock and applies the same to the interval width counter 2. The null interval width base counter 4 stores standard data about a null symbol interval width in accordance with transmission modes. The interval width evaluator 5 compares data about an interval width detected in the interval width counter 2 with the standard data about the null width stored in the null interval width base counter 4 to determine whether or not a currently inputted symbol is a null symbol. The synchronous signal generator 6 generates a synchronous signal which is synchronous with the null symbol when the interval width evaluator 5 determines that the corresponding symbol is the null symbol.
Operations of the null symbol detection device illustrated in FIG. 1 will be specifically described by taking detection of null symbol in European Digital Audio Broadcasting (DAB) standard (ETS300401) for digital broadcasting as an example. In European DAB standard, four transmission modes, i.e., modes 1 to 4 are provided. For example, assume that it is known that a currently received transmission mode is the mode 1. In the mode 1, a null symbol is transmitted at a period of 96 msec. The width of the null symbol is 1.297 msec.
When a baseband signal is inputted to the level detector 1, the output of the level detector 1 is in an H-level at the position that a null symbol starts. Then, the interval width counter 2 starts its count operation. When the null symbol ends, the output of the level detector 1 is in an L-level and the interval width counter 2 stops the count operation. The interval width evaluator 5 determines whether or not the interval width detected by the interval width counter 2 is within a standard range of the null symbol width for the mode 1 stored in the null interval width base counter 4. If the detected interval width is within the standard range, it is determined that the null symbol is provided. At this period, the synchronous signal generator 6 outputs a synchronous signal which is synchronous with the null symbol. By such operations, the null symbol is detected and the transmission mode can be identified. When a plurality of modes are used, the modes are successively detected and, then, it is determined whether or not a current signal is within the standard range of the null symbol width for the corresponding mode.
Assume that a terrestrial digital broadcast in European DAB standard is received in a mobile manner. For example, in an in-vehicle receiver, a direct wave and a reflection wave complicatedly interfere with each other, so that a multipath is generated. Further, a fading may occur by movement of the on-vehicle receiver. Such factors may vary greatly the level of a received signal. The level detector 1 compares the signal level of a baseband signal with a predetermined threshold. At this time, the level detector 1 may output an H-level signal at positions other than the null symbol.
In accordance with the conventional null symbol detection device with the above-described structure, the interval width counter 2 detects the interval width when the level detector 1 outputs an H-level signal. The interval width evaluator 5 compares this interval width with a null symbol width for a predetermined mode. Thus, the null symbol detection device distinguishes a trough in a signal level generated by fading from an original null symbol.
In accordance with a detection method by the conventional null symbol detection device, however, the transmission mode of a received signal must be known in advance. Further, determination of transmission mode of the received signal and detection of the null symbol cannot be performed at the same time. Further, the width of trough in a signal level generated by fading may approximately coincide with the width of the null symbol. Then, an interval which is not for the null symbol may be erroneously detected as the null symbol. Moreover, if the null symbol coincides, in view of time, with the trough of the fading, the width of a trough in the signal level is widened significantly as compared with the width of the original null symbol, so that the null symbol may be missed in a desired transmission mode.